Antenna diversity is an approach to cope with multipath fading of received signals in wireless communication systems, as encountered for example in radio frequency (RF) communication inside or between buildings, where multipath transmission may cause destructive interferences and therefore reduce the range for reliable communication.
As shown for example in FIG. 1, a communication system 10 without antenna diversity may comprise a transmitter (TX) or sender 12 with a single transmitter-side antenna 14, and a receiver (RX) 16 with a receiver-side antenna 18. A signal may be a change of a physical quantity carrying information, for example an electromagnetic wave. A signal may for example be a radio frequency signal or an optical signal. The signal, i.e. for example the electromagnetic waves comprising an information to be transmitted from a sender or transmitter to a receiver, radiated from the transmitter-side antenna 14 may for example be reflected at a first wall 20 and a second wall 22, therefore generating in the shown example three transmission paths for transmission of three instances or multipath representations of the signal 24, 26, 28 potentially interfering with each other at the receiver-side antenna 18.
For improved received signal quality, the signal 24, 26, 28 may be transmitted using more power or the receiver 16 may be configured for providing more received signal sensitivity. However, this may result in very strict transmission link margins.
As shown for example in FIG. 2, a different communication system may be a diversity antenna system 30 comprising a transmitter 12 having a single transmitter-side antenna 14 and a diversity receiver 32 having two or more receiver-side antennas 34, 36 which may receive different multipath signals 24, 26, 28; 38, 40, 42 and therefore resulting signals received at the different antennas may not be affected simultaneously by the same interferences. The signals received by each antenna 34, 36 are then compared, and the antenna providing the better reception or the best received signal quality (for example higher signal-to-noise ratio) as measured by the receiver is selected.
For bi-directional communication, transceivers may be used providing both transmitter and receiver functionality. A typical diversity transceiver is shown in FIG. 3. The transceiver 44 may comprise a transmitter 46 connected to a transmit antenna 48 and two or more receivers 50, 52, each connected to a different receive antenna 54, 56. In the shown example, 3 pins 58, 60, 62 are used (2 for RX, 1 for TX) for antenna connection. A microcontroller unit (MCU) 64 may be connected to the transmitter for providing the data to be transmitted and may be connected to the receivers for receiving data and signal level values on the receive antennas and evaluate a signal strength or signal level and decide which antenna to use. For using a MCU for managing the antenna diversity function, received signals may be converted from the received analog representation to a digital representation using an analog-to-digital converter (not shown). During signal reception, the MCU may be in an ON-mode, i.e. the MCU may be awake.
In telecommunications, low-power communication devices or Short Range Devices (SRD) may also be subject to multipath fading. A SRD is a restricted radiation device, for example a wireless microphone, a radio-controlled garage door opener, or other device, for example a medical or automotive device, using wireless communication, wherein a distance between transmitter and receiver is typically only a few meters. Many of these devices for example communicate using the industrial, scientific and medical (ISM) radio bands, which were originally reserved internationally for the use of RF electromagnetic fields for industrial, scientific and medical purposes other than communications. Communications equipment must usually accept any interference generated by ISM equipment. ISM bands are for example in the range of 315 MHz, 433 MHz or 868 MHz. Short range devices are often small, which may restrict the area available for several antennas or may limit the available power for a receiver side digital processing unit and connected analog-to-digital converter for diversity receiver signal processing.